This invention relates to preventing corrosion of susceptible substrates as ferrous metals and includes, in particular, resins that retard corrosion of such substrates even when formulated in primer compositions that are free of such inhibiting pigments as chromates.
The corrosion of steel immersed in an aqueous environment can often be slowed to an acceptable rate by the introduction of corrosion inhibitors soluble in the aqueous medium. In the case of steel structures subjected intermittently to a corrosive environment, or under conditions which preclude effective incorporation of a corrosion inhibitor into the corrosive medium, protective organic coatings are often used. One of the major functions of these coatings is to act as a reservoir for sparingly soluble inorganic corrosion inhibitors which act to slow the rate of corrosion at coating defects and other incipient anodic corrosion sites. Among the most effective inhibitors are inorganic chromates whose safety has been questioned by some. While the elimination of these pigments may therefore be desirable, it is found that in the absence of effective corrosion inhibiting pigments extensive adhesion failure adjacent to coating defects and flaws can take place, resulting ultimately in the uncontrolled spread of corrosion. The principal cause of corrosion-induced adhesion failure is generally thought to be displacement and/or degradation of the coating resin by cathodically produced hydroxide ion. Corrosion inhibitors suppress this effect by slowing the overall rate of corrosion, limiting the amount of alkali produced. In the absence of corrosion inhibitors it is therefore necessary that the coating resin be exceptionally resistant to displacement and/or degradation by corrosion-produced hydroxide.
A principal objective is, therefore, the provision of resins suitable for use in organic coatings and additionally highly resistant to alkali degradation. A further objective is the provision of organic coating formulations which enable ferrous substrates to be protected from corrosion without the incorporation of possibly deleterious corrosion inhibitive pigments. It is further desired that the organic coatings be suitable for use as primers on automotive steel substrates and that the application and handling characteristics, including shelf stability, be compatible with conventional commercial paint operations and practices.